Your local university should offer forensic science degrees. Get a degree in that then you have two choices. 1. apply at your local crime lab 2. Take the civil service test for entry into the police or fire department. Go through the academy and then work in the field.

Since you specifically ask about "cold cases," those cases are usually assigned to either the very best investigators, or investigators being punished, depending on that department's attitude toward "cold cases." Cold cases are seldom solved.